To detect an open/close state of a fluid flow path in a manual valve, there has been known a device which detects an upper end position and a lower end position of a valve stem using limit switches. Although such an opening degree detection device can detect a fully open state or a fully closed state, the opening degree detection device has a drawback that the opening degree detection device cannot detect an intermediate state such as the opening degree of 5% or the opening degree of 95%. In view of the above, patent literature 1 proposes an opening degree detection device for a manual valve where, using a displacement sensor fixed to a valve body by way of a mounting fitting and a tapered cylindrical target integrally mounted on a handle and a valve stem, the opening degree of the fluid flow path is detected based on a measured value of the displacement sensor which is arranged so as to face a detection-use inclined surface (tapered cylindrical surface) of the target which moves along with the upward and downward movement of the valve stem.